1. Field of the Invention
This invention relates to a polymer blend composition. More particularly, this invention relates to a polymer blend composition comprising a selectively hydrogenated block copolymer, a vinyl aromatic copolymer, polycarbonate and optionally a saturated diblock copolymer.
2. Description of the Prior Art
Vinyl aromatic resins, such as polystyrene or high impact polystrene have been found to be useful in thermoplastic molding compositions. However, such vinyl aromatic resins have poor heat distortion and impact resistance. One approach to improve these property deficiencies involves copolymerizing the vinyl aromatic with an .alpha., .beta.-unsaturated cyclic anhydride, to form copolymers such as poly(styrene-maleic anhydride). Although such copolymers have improved heat resistance, the overall balance of properties is still inadequate.
In order to further improve the properties of such vinyl aromatic copolymers, various other polymers have been blended with the copolymer. For example, blends of nitrile rubber and styrene-maleic anhydride copolymers are disclosed in U.S. Pat. Nos. 2,914,505 and 3,641,212. Blends of styrene-maleic anhydride copolymers with radial styrene-diene block copolymers and an optional polyphenylene ether resin are disclosed in U.S. Pat. No. 4,097,550. Still further, blends of styrene-maleic anhydride copolymers, hydrogenated styrene-diene block copolymers and optional polyphenylene ether resins are disclosed in U.S. Pat. Nos. 4,124,654 and 4,243,766. Such polymer blend compositions are still not ideal.
Blends of styrene-maleic anhydride copolymers, hydrogenated styrene-diene-styrene copolmers and thermoplastic polyesters disclosed in U.S. Pat. No. 4,377,647 show good properties especially when blended with oils.
U.S. Application Ser. No. 502,407 discloses blends of triblock copolymer, a vinyl aromatic copolymer, a polyester and a diblock. These blends showed good properties but exhibited excessive shrinkage after molding and painting.
A characteristic property of any thermoplastic molding composition is mold shrinkage. Parts molded from some polymers also exhibit shrinkage after the molding process when exposed to higher temperatures for some period of time. This shrinkage can pose of significant problem.
Experimentation with blends containing styrene-diene-styrene copolymers such as those mentioned above has shown the blends to exhibit a constant volume shrinkage in the injection/compression mold orientation directions when allowed to reach equilibrium at 250.degree. F.
Blends of these block copolymers as described above have been found to possess properties desirable for painted exterior automotive application. The automotive industry has been using molds and tools cut for thermoplastic olefin (TPO) materials. These materials have an overall shrinkage of about 18 mils per inch and the molds have been designed to accomodate this.
In order to allow use of existing TPO molds a material with an overall shrinkage when allowed to reach equilibriums at 250.degree. F. of about 18 mils per inch is needed. The overall shrinkage of a painted, injection molded automotive part consists of two components; that occuring during the injection molding process (mold shrinkage) and that occuring during the paint baking process (oven shrinkage). In order to achieve a particular part size, tooling must be designed to accomodate both of the shrinkages. The typical equilibrium shrinkage characteristics of typical exterior automotive materials are shown in Table I.
TABLE I ______________________________________ SHRINKAGE CHARACTERISTICS OF EXTERIOR AUTOMOTIVE MATERIALS Mold Oven Total Shrinkage Shrink Shrink ______________________________________ BF Goodrich Estane 58130 8 Mils 4 Mils 12 Mils (Thermoplastic urethane) Mobay Merlon 40 6 Mils 4 Mils 10 Mils (Polycarbonate) General Electric 17 Mils 12 Mils 29 Mils Xenoy 1200 (Alloy) General Electric 20 Mils 6 Mils 26 Mils Valox 310 (polybutylene- terethalate) Republic 3041 13 Mils 5 Mils 18 Mils (thermoplastic olefin) Shell Kraton 7827 12 Mils 3 Mils 15 Mils (thermoplastic olefin) ______________________________________
Applicants have discovered that by adding polycarbonate to the blends replacing the PBT, the equilibrium shrinkage of the blends after molding and painting could be decreased to about 18 mils per inch total with a wide range of component ratios. Excellent physical and painting properties were also maintained with the blends making them good candidates for exterior decorated automotive parts.